To facilitate construction of strains producing elevated amounts of the proteins involved in covalent modification of glutamine synthetase (GS), we have characterized mutants of Escherichia coli which require glutamine for growth. Using a class of mutants (glnD) that lack uridylyltransferase (UTase) activity, we have screened a collection of 2,000 E. coli strains carrying hybrid ColEl plasmids (L. Clarke and J. Carbon (1976) Cell 9, 91-99) for those capable of correcting the glutamine requirement of the g1nd strains. Two hybrid plasmid-containing strains capable of complementing the g1nd mutations were found to overproduce UTase by 14- to 25-fold. In both of these strains the increase in UTase was paralleled by a concomitant increase in the levels of uridylyl-removing (UR) enzyme. Strain JA200/pLC 38-39, which produces 25-fold the normal level of UR-UTase, will be used for the isolation of this (these) enzyme (s).